Replacing a hydraulic o-ring without reading the old one means you will probably be back. The failure pattern tells you what to change.
Extrusion
Looks like: the ring has a flattened tail or flange pressed out on one side, as if squeezed through a gap.
Cause: pressure forced the elastomer into the clearance between the mating parts. Usually the ring is too soft — 70 durometer where 90 belongs — or the clearance gap is too large from worn components.
Fix: correct durometer, or a backup ring. This is the single most common hydraulic o-ring failure.
Nibbling
Looks like: small bites or chunks missing from one edge.
Cause: repeated extrusion and retraction as pressure cycles. The ring is pushed into the gap, then pulled back, and each cycle tears a little away.
Fix: same as extrusion — harder material or a backup ring.
Spiral failure
Looks like: a cut spiralling around the ring's circumference, sometimes almost a corkscrew.
Cause: the ring twisted rather than sliding during dynamic movement. Common on long-stroke cylinders and where installation was dry.
Fix: lubricate on assembly, check surface finish, and consider a harder ring.
Compression set
Looks like: flattened on two sides, permanently. It no longer looks round in cross-section.
Cause: heat, age, or over-compression. The elastomer has stopped recovering.
Fix: a material rated for the temperature, and check that the groove is not over-squeezing it.
Explosive decompression
Looks like: blisters, splits or internal cracking that appears to come from inside the ring.
Cause: gas absorbed into the elastomer under pressure expands rapidly when pressure drops.
Fix: a material specified for the service and, where possible, slower depressurisation.
Chemical attack
Looks like: swelling, softening, or conversely hardening and cracking, usually uniform around the whole ring.
Cause: wrong elastomer for the fluid.
Fix: see the material guide.
The recurring theme
Most hydraulic o-ring failures come back to durometer. Hydraulic service generally wants 90; general purpose is 70; they look identical in the hand. Our ORB, ORFS and flange kits are 90 durometer for that reason.
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